This invention relates in general to optical gyroscopes and, in particular, to fiber-optic gyroscopes. The invention relates especially to phase modulation and signal processing in fiber-optic gyroscopes.
The disadvantages of mechanical gyroscopes having spinning masses are well known. For example, they suffer from relatively long "spin-up" times measured in minutes or hours. They contain large numbers of precision parts and are expensive and time-consuming to produce. They are dynamically sensitive to shock and vibration due to mechanical compliance and spin synchronous frequency response phenomena. When they are supported by ball bearings, they may have a relatively short useful life. In contrast, fiber-optic gyroscopes offer many advantages. They turn on and off instantly. They have a long shelf life and can be small and rugged. Since they do not require expensive gimballed mounting systems, they are relatively low in cost.
In the basic fiber-optic gyroscope, an optical beam is split into two equal beams and coupled in opposite directions through a multiturn fiber optic coil. The two counter-propagating beams are recombined and coupled to a photodetector such that the two beams traverse identical closed optical paths. When the fiber coil is rotated about its axis relative to inertial space, Sagnac phase shifts of equal magnitude and opposite sign are produced in the two beams. This Sagnac phase shift is linearly proportional to this rotational rate and is detected as a change in the interference pattern at the photodetector.
As is evident from the article entitled "Fiber-optic gyroscopes" by Kim and Shaw, IEEE Spectrum, March 1986, the fiber-optic gyroscope, at present, is primarily a laboratory device and many investigators are working on perfecting the fiber-optic gyroscope for use in a practical environment. One problem in providing a practical rate sensor based on the Sagnac effect has been the signal processing required to provide a useful rate output. One approach to this problem involves sinusoidally modulating the counter propagating optical beams near one end of the fiber coil to provide a nonreciprocal phase shift in the beams and then demodulating the output of the photodetector to provide a linear signal proportional to the rotation rate. However, heretofore, the implementation of this approach has not been entirely satisfactory in providing a practical device. Accordingly, the present invention is directed to providing a phase modulation and signal processing technique which enables the construction of a practical fiber-optic rate sensor using a Sagnac ring circuit as the rate sensing means.